You do know lore can move and change and progress right? Prior of wow, it would had never been imagined that undead and elf would be part of Horde.

I know this, but undead or elves didn't enslave other races and called them "lesser beings". Mogu are very pridefull, they would never join other races. They're like Garrosh, and we are disposing off Garrosh at the end of MoP. Playable Mogu didn't make any sense.

Also the Mogu are an evil race, they would never care about petty Horde v. Alliance drama when all they care about is reclaiming their empire and ruling all of us 'lesser races'. Furthermore, can you imagine them teaming up side by side with their former slaves the Pandaren?